1. Field of the Invention
The present invention relates to method and apparatus for applying a secondary layer or coating on surfaces of wet formed board materials.
2. Description of the Prior Art
It is known to coat pressed board to achieve various advantages over non-coated boards. For example, combustible board comprising cellulosic fibers may be coated with a fire resistant layer containing asbestos, mica or other fire resistant materials. Also, a secondary layer may be applied as a finishing coating to give an inexpensive substrate an expensive visual appearance, as, for example a wood finish on a mineral fiber board. Secondary coatings are also applied to moisture-proof inexpensive hydrophillic substrates. For example, a phenolic coating may be applied to a substrate containing starch binder. Structurally, a moisture-resistance outer layer will prevent or minimize warping of a board having a hydrophillic substrate. Other advantages to coating pressed board are apparent to those skilled in the art.
It is known to apply the secondary layer or coating by depositing slurry material directly on the substrate and screeding and pressing the deposited slurry. The product is then dried and the surface layer becomes adhered to the substrate. U.S. Pat. Nos. 3,077,945 and 3,270,476 disclose the application of a secondary layer on a board surface by a roller. The present invention is a significant advance over known apparatuses and methods in that coatings of uniform thickness, even relatively thick coatings, may be applied to board surfaces using slurries of varying consistencies, even slurries which are quite liquid.